The present invention relates to a protection device for a semiconductor switch for protection against voltage surges as well as to a method for operating a protection device for a semiconductor switch in order to protect against voltage surges.
The use of semiconductor switches as electrical switching elements is known. Hence, voltage surges occur upon opening the switching elements, in particular in power electronic systems, such as, for example, in DC-DC converters or in inverters, on account of the always present leakage inductances. If the current flow through a semiconductor switch is interrupted, the voltage at the semiconductor switch between collector and emitter then increases to a value which can be greater than the supply voltage. If the resulting voltage exceeds the maximum reverse voltage of the semiconductor switch, said semiconductor switch can then be destroyed.
FIG. 1 shows a simple protection circuit for protecting a semiconductor switch T100 against voltage surges. The semiconductor switch T100 is thereby actuated by means of a gate resistor RG of a gate drive circuit 400. An avalanche diode D100 is disposed between the collector C and the gate G of the semiconductor switching element T100. If the voltage between collector C and emitter E at the semiconductor switch T100 exceeds a value predetermined by the avalanche diode D100, said avalanche diode D100 then starts conducting. The semiconductor switch T100 opens thereupon at least partially and therefore becomes slightly conductive. The shutdown speed di/dt of the collector current of T100 is thus reduced to a point such that the collector-emitter voltage is limited to an admissible level.
Due to the static embodiment of this circuit, the shutdown point must lie above the maximum possible supply voltage. Otherwise the danger exists that the semiconductor switch passes over into a continuous operation which would lead to the thermal destruction of said semiconductor switch.
The German patent publication DE 10 2010 008 815 A1 discloses a surge protection for a semiconductor. In this case, the protection circuit for the semiconductor is only then activated if a switch operation is actually performed on the semiconductor switch. After being correspondingly enabled, the protection circuit here too only then begins to respond if the collector-emitter voltage exceeds a threshold value above the maximum supply voltage.
The need therefore exists for an improved surge protection for semiconductor switches. In particular, there is a need for a surge protection for semiconductor switches which responds especially quickly. In addition, there is also a need for a surge protection in which a dangerous continuous operation can be reliably prevented.